Quiet Power: PCB Fixtures for Power Integrity

Power integrity components—such as bypass capacitors, inductors, ferrite beads, or other small discrete components—can be characterized in fixtures. There is a wide range of fixtures available, from the professional and very accurate [1] to the home-made and very crude [2]. In between these extremes, you will find various printed circuit board fixtures, such as the decoupling test board kit shown in Figure 1 [3] or the RF experimenter board set shown in Figure 2.

The Picotest boards come with Touchstone files for de-embedding the measured data. I particularly like this kit because it has separate small boards with solder pads specifically for a wide range of surface-mount component sizes, including reverse geometry capacitors and some medium-size bulk capacitors. The range starts with the 0201 size and includes 0204 and 0612 reverse-geometry sizes. Large-size polymer capacitors can be tested on the D-size (7343) fixture; for surface-mount cylindrical capacitors, we get 5-mm and 8-mm sites. To test filter structures, two of the smaller sizes (0402 and 0603) also have a generic T scheme: pads for two components in the series path and for one component in the shunt path. There is a dedicated site for single-body 0402- X2Y filter elements. This test board kit also has a single-piece open-short-load impedance reference board; you can find it in the oversized lower-middle compartment in Figure 1. You can also build fixtures straight out of small coaxial connectors [4].

With all of these fixtures, we need to keep in mind that the current path around the device we characterize does not necessarily match the current path that is created by the layout and stackup in our final application. In the solder-wick fixture, the current path is highly uncertain; the shape of the flexible connections will vary depending on how we achieve the pressure-mount connection. With the fixtures built entirely out of SMA connectors, we have a fixed geometry for the connector pieces, but there are no dedicated pads to solder the parts down, so the actual current path depends on how we solder the DUT between the center pins and outer frame. This means that the extracted mounted inductance values will need to be used with some caution.

To read this entire column, which appeared in the February 2020 issue of Design007 Magazine, click here.

2020

Power-integrity components—such as bypass capacitors, inductors, ferrite beads, or other small discrete components—can be characterized in fixtures. Istvan Novak discusses the wide range of PCB fixtures available for power integrity.

2018

A year ago, I introduced causal and frequency-dependent simulation program with integrated circuit emphasis (SPICE) grid models for simulating power-ground plane impedance. The idea behind the solution was to calculate the actual R, L, G, and C parameters for each of the plane segments separately at every frequency point, run a single-point AC simulation, and then stitch the data together to get the frequency-dependent AC response. This month, I will demonstrate how that simple model correlates to measured data and simulation results from other tools.

2017

Causal and frequency-dependent models and simulations are important for today’s high-speed signal integrity simulations. But are causal models also necessary for power integrity simulations? When we do signal integrity eye diagram simulations, we define the source signals, so if we use the correct causal models for the passive channel, we will get the correct waveforms and eye reduction due to distortions on the main path and noise contributions from the coupling paths. Istvan Novak explains.

2016

A year ago, my Quiet Power column described the possible large loss of capacitance in multilayer ceramic capacitors (MLCC) when DC bias voltage is applied. However, DC bias effect is not the only way we can lose capacitance. Temperature, aging, and the magnitude of the AC voltage across the ceramic capacitor also can change its capacitance.

2015

There is a well-established theory to design stable control loops, but in the case of power converters, we face a significant challenge: each application may require a different set of output capacitors coming with our loads. Since the regulation feedback loop goes through our bypass capacitors, our application-dependent set of capacitors now become part of the control feedback loop. Unfortunately, certain combination of output capacitors may cause the converter to become unstable, something we want to avoid. This raises the need to test, measure, or simulate the control-loop stability. Istvan Novak has more.

The density of multilayer ceramic capacitors has increased tremendously over the years. While 15 years ago a state-of-the-art X5R 10V 0402 (EIA) size capacitor might have had a maximum capacitance of 0.1 uF, today the same size capacitor may be available with 10 uF capacitance. This huge increase in density unfortunately comes with a very ugly downside. Istvan Novak has more.

2014

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).

In his last column, Istvan Novak looked at the importance of properly terminating cables even at low frequencies and also showed how much detail can be lost in PDN measurements when bad-quality cables are used. This month, he analyzes a step further the shield in cables.

2013

In his August column Istvan Novak looked at the importance of properly terminating the cables that connect a measuring instrument to a device under test. He writes that we may be surprised to learn that even if the correct termination is used at the end of the cable, the measured waveform may depend on the quality of the cable used.

In high-speed signal integrity measurements, the first rule is to properly terminate traces and cables. However, many PDN measurements may be limited to lower frequencies, such as measuring the switching ripple of a DC-DC converter. Do you really need to terminate measurement cables if the signal you want to measure is the switching ripple of a converter running at 1 MHz?

PDN noise can be measured in a variety of ways, but measuring across a capacitor will attenuate the high-frequency burst noise. Keep in mind that by measuring across a capacitor, the converter output ripple reading could be several times higher--or many times smaller--than the actual ripple across our loads.

To use bypass capacitors properly, any designer must understand ESR (effective series resistance). A designer must understand what it means and how to read the ESR curve in measured or simulated plots.

2012

Recently, one of Istvan Novak's friends asked him about the preferred method of probing a power distribution network: "Which probe should I use to measure power plane noise?" Although, as usual, the correct answer begins with "It depends," in this case the generic answer is more clear-cut: For many PDN measurements, a simple passive coaxial cable is better.

Istvan Novak takes a look at an award-winning paper presented at DesignCon 2012, and he discusses the apparent disappearance of power planes from PCBs. In the future, the need for power planes may diminish or go away altogether. The change is already under way, and power planes, full-layer planes in particular, are disappearing fast from printed circuit boards.

My friend Greg recently asked me, "If I add surface-mount capacitors to a bare pair of planes, I am told that the resonant frequency will drop. On the other hand, someone with expertise is telling me that this is not the case. What would you expect to see?" As happens many times, both observations have elements of the truth in them, and a third scenario is not out of the question.

2011

Last month, we learned how we can determine the grid equivalent circuit parameters for a plane pair. You may wonder: Is it better to use LC lumped components in the SPICE netlist or to make use of SPICE's built-in transmission line models? In short, we can use either of them, but we need to set up our models and expectations correctly.

There are several excellent commercial tools available for simulating power distribution planes. However, you don't need a commercial tool to do simple plane analysis. You can, for instance, write your SPICE input file and use the free Berkeley SPICE engine to get result. If you want to do your own plane simulations, there are a couple of simple choice.

We know that in signal integrity, the relative dielectric constant (Dk) of the laminate is important. Dk sets the delay of traces, the characteristic impedance of interconnects and also scales the static capacitance of structures. Is the same true for power distribution? The answer is yes, but for power distribution all this matters much less.

2010

For this column, I will take a quick detour from the series on the inductance of bypass capacitors. I will devote this column to a few comments about via placement and its potentially detrimental impact on signal and power integrity when antipads heavily perforate planes.

In Part III of a series, we'll take a look at loop or mounted inductance. Loop inductance is important, for instance, when we need a reasonably accurate estimate for the Series Resonance Frequency (SRF), or for the anti-resonance peaking between two different-valued capacitors or between the capacitor's inductance and the static capacitance of the power/ground planes it connects to.

We finished the last Quiet Power column with a few questions about the inductance of bypass capacitors: Why do different vendors sometimes report different inductance values for nominally the same capacitor? Start by asking the vendors how they obtained these inductance values.

Why is power integrity design more difficult than signal integrity design? Reasons abound, and unlike SI, we've only begun to study PI. Collective wisdom and experience gained over the coming years will help to alleviate the pain somewhat, but we should expect the challenge to stay with us for some time.

At low and mid frequencies, where the self-impedance of a DUT may reach milliohm values, a fundamental challenge in measurement is the connection to the DUT. Unless we measure a single component in a well-constructed fixture, the homemade connections from the instrument to the DUT will introduce too much error. What's the solution? By Istvan Novak.

In my last column, I showed that the piecewise linear Bode plots of various PDN components can create peaking at some interim frequencies. Today, I must cover peaking in more detail, because, even today, certain articles, books and CAD tools provide the wrong answers to this problem.